Assessing the risk of primary amoebic meningoencephalitis from swimming inthe presence of environmental Naegleria fowleri

Free-living Naegleria fowleri amoebae cause primary amoebic meningoencephal itis (PAM). Because of the apparent conflict between their ubiquity and the rarity of cases observed, we sought to develop a model characterizing the risk of PAM after swimming as a function of the concentration of N. fowleri . The probability of death from PAR4 as a function of the number of amoebae inhaled is modeled according to results obtained from animals infected wit h amoeba strains. The calculation of the probability of inhaling one or mor e amoebae while swimming is based on a double hypothesis: that the distribu tion of amoebae in the water follows a Poisson distribution and that the me an quantity of water inhaled while swimming is 10 ml. The risk of PAM for a given concentration of amoebae is then obtained by summing the following p roducts: the probability of inhaling n amoebae x the probability of PAM ass ociated with inhaling these n amoebae. We chose the lognormal model to asse ss the risk of PAM because it yielded the best analysis of the studentized residuals. Nonetheless, the levels of risk thereby obtained cannot be appli ed to humans without correction, because they are substantially greater tha n those indicated by available epidemiologic data. The curve was thus adjus ted by a factor calculated with the least-squares method. This provides the PAM risk in humans as a function of the N. fowleri concentration in the ri ver. For example, the risk is 8.5 x 10(-8) at a concentration of 10 N. fowl eri amoebae per liter.